1. Field of the Invention
The invention relates to display systems, and more particularly, to an image processing device and method thereof.
2. Description of the Related Art
Traditionally, analog front end (AFE) circuits are applied to two categories of display systems. First, an AFE circuit applied to a liquid crystal display (LCD) controller without a decoder is used to receive three analog image signals R, G, B from a display card (e.g., VGA card) of a computer system. Second, an AFE circuit applied to a video decoder is used to receive a signal from a tuner or a DVD player. Where the signal can be divided into three video formats as follows. The first is a composite video signal, often called a CVBS signal, which combines the luminance (Y) and chrominance (C) signals into a single channel. The second is a separate video signal separating the luminance (Y) and the chrominance (C) signals. The third is a component video signal which is split into three separate signals Y, Pr, Pb.
Conventionally, AFE circuits perform AC-coupling and clamping. Take three analog image signals R, G, B sent from a display card for example. Since the DC components of these signals are unable to pass through AC-coupling capacitors, the AFE circuits usually utilize clampers to restore the DC voltage levels of the analog image signals. In general, AC-coupling and clamping are performed in order to restore the DC voltage levels of the analog image signals; however, it usually takes a while for the restored DC voltage levels to settle, thereby unable to reflect an immediate change in the DC voltage levels of the analog image signals.
According to video formats of the analog image signals received by the AFE circuits, the structures of clampers are basically divided into two types as follows.
The first is a voltage-type clamper, which is generally employed to process analog image signals R, G, B, or a component video signal (Y, Pr, Pb). The voltage-type clamper includes a switch SW in connection with a restored voltage Vres and the operation of the switch SW is controlled by a clamping signal. The restored voltage Vres will charge an AC-coupling capacitor up to a voltage level of Vres as the switch SW is turned on.
The second is a current-type clamper, which is generally employed to process a component video signal (Y, Pr, Pb), a composite video signal or a separate video signal (Y, C). The current-type clamper is formed by two series connected switches SW1, SW2, which are respectively connected with two current sources I-up, I-dn. The operation of the switch SW1 is controlled by a clamping signal clamp_up while the operation of the switch SW2 is controlled by a clamping signal clamp_dn. A circuit designer can establish a DC voltage at a junction of the two series connected switches SW1, SW2 by adjusting pulse widths and conductive periods of the clamping signals (clamp_up, clamp_dn). Finally, the DC voltage at the junction of the two series connected switches SW1, SW2 charges the AC-coupling capacitor up to a desired voltage level.
In fact, no matter which type of clamper is selected, a problem that a change in the DC voltage levels of the analog image signals can not be reflected immediately will arise as long as the clamper operates in an image processing device with AC-coupling function. Further, if the analog image signals contain a Sync_on_Green (SOG) signal, the circuit will include an additional branch having an AC-coupling capacitor and a SOG circuit. After receiving the SOG signal, the SOG circuit restores the DC voltage level of the SOG signal and then extracts a sync signal from the SOG signal by using an internal auto-clamper. Where the extracted sync signal is a composite sync signal HS+VS containing a horizontal sync (HS) signal and a vertical sync (VS) signal. Likewise, with AC-coupling and clamping, the SOG branch has the same problem that a change in the DC voltage levels of the SOG signals can not be reflected immediately.
In addition to performing AC-coupling and clamping, conventional AFE circuits usually use input buffers to buffer restored DC voltage levels of the analog image signals and prevent restored DC voltage levels of the analog image signals from running. The input buffers, formed by analog circuits, have an input impedance of infinity and an output with high driving capability. However, installing an input buffer in an AFE circuit causes a problem which it is not easy to design an input buffer with high input impedance.
Accordingly, what is needed is an image processing device to address the above-identified problems. The invention addresses such a need.